In general, inks for a ball point pen have so far been classified into a low-viscosity, water based ink for a ball point pen which contains a solvent comprising a water based solvent such as water and has an ink viscosity of 10 mPa.multidot.s or less and a high-viscosity, oil based ink which contains a solvent comprising an oil based solvent such as mineral oil, a polyhydric alcohol, a fatty acid and cellosolve and has an ink viscosity of 1000 to 20000 mPa.multidot.s.
A ball point pen using an oil based ink for a ball point pen has such structure that the ink adhered on a ball through an ink reservoir having a small diameter is transferred onto a paper surface by rotation of the ball and only the transferred portion of the ink is fed from the reservoir to the ball.
A ball point pen using a water based ink for a ball point pen has such structure that the ink is fed to a ball surface and a paper surface by virtue of capillary action of a feed prepared by binding fine fibers.
While a water based ink and an oil based ink for a ball point pen each have advantages, they have various problems as well. For example, the water based ink for a ball point pen has a low viscosity, and therefore only if the point of the ball point pen is simply brought into contact with paper, capillary action is exerted on the contact point thereof to feed the ink. The ink makes it possible to write fine lines on a paper surface without requiring so higher writing pressure and scarcely produces splitting, starving and blobbing. On the contrary, storing the ink directly in the ink reservoir allows the ink to seep due to vibrations, impacts and a rise in an open air temperature as well as to make the amount of the ink to be fed to the ball unstable. Accordingly, complicated structure having a feed prepared by binding fine fibers is required. Further, involved is the problem that it is difficult to observe the remaining amount of the ink.
On the other hand, an oil based ink for a ball point pen is characterized by that because of a high viscosity thereof, blobbing of the ink from the pen tip can be prevented and the ink can be stored directly in the ink reservoir having a small diameter, so that the structure of the ball point pen can be simplified and that the remaining amount of the ink can be observed by using a transparent material for the ink reservoir. On the contrary, there are involved the problems that since the ink is transferred only on a paper surface contacted with the rotating ball, splitting and starving are liable to be caused if the ball rotates unstably and that since the ink hardly penetrates into a paper surface, blobbing which causes stain with the untransferred ink is apt to be caused.
In recent years, in order to solve such problems, developed are water based inks for a ball point pen in which a gelatinizer and a water-soluble paste are added to impart a specific viscosity characteristic to the ink (hereinafter referred to as a pseudoplastic water based ink).
A ball point pen using this pseudoplastic water based ink is reduced in an ink viscosity by shear force which is applied to the ink by rotation of the ball at the tip point when writing and can write as smoothly as a ball point pen using a water based ink and make good lines on a paper surface. Further, blobbing of the ink from the pen point can be prevented because the ink has a high viscosity when the pen is not used for writing. The ink can be stored directly into the ink reservoir, so that the structure of the ball point pen can be simplified, and the use of a transparent material for the ink reservoir makes it possible to check the remaining amount of the ink.
Thus, the use of the pseudoplastic water based ink for a ball point pen provides a writing instrument having both characteristics of a ball point pen using a water based ink and a ball point pen using an oil based ink. However, a water based ink contains water as a principal solvent, and if water contained therein is evaporated, it has a difficulty in writing, because dissolved substances and mixtures contained in the ink are concentrated, deposited, dried and solidified at the point of the writing instrument to cause clogging and a rise in the viscosity of the ink. Accordingly, it has so far been proposed to add a less volatile agent for solubilizing a dye or a less volatile aid for dissolving a dye, such as urea, thiourea, polyhydric alcohols or derivatives thereof, tetrahydrofurfuryl alcohol, 4-methoxy-4-methylpentane-2-one, ethylene oxide adducts of p-toluenesulfonamide, thiodiethanol and alkanolamine, and sorbitol.
However, satisfactory water based ink compositions have not been obtained because inks blended with the various additives described above do not have a sufficient effect on dryout resistance and further have the problem that the viscosities are increased so as to bring about inferior follow-up of ink and that the additives have unsuitably toxicity.
Further, dextrin and maltodextrin which have reducing end groups and are represented as starch decomposition products or glucose polymers are added. However, while they have dryout resistance at a pen tip, blending them into an ink promotes the reaction between them and amino groups in a pH controlling agent and a water-soluble resin contained in the ink components (Maillard reaction) with the passage of time because they are nonreduced sugars, so that brought about is the problem that a pH of the ink is reduced.
The present invention has been made in order to solve the conventional problems described above, and an object of the present invention is to provide a water based ink composition for a writing instrument which has excellent dryout resistance and is safe and in which the viscosity and pH do not vary with the passage of time, particularly a pseudoplastic water based ink for a ball point pen.